Enhanced coherency of thermal emission by coupled resonant cavities supporting surface waves Nir Dahan, Avi Niv, Gabriel Biener, Yuri Gorodetski, Vladimir Kleiner, and Erez Hasman Optical Engineering Laboratory, Faculty of Mechanical Engineering & Russel Berrie Nanotechnology Institute, Technion − Israel Institute of Technology, Haifa 32000, Israel ABSTRACT Surface waves have been shown to play a key role in spontaneous thermal emission in the near-field as well as the coherence and the polarization properties of the nonradiative field. The near-field coherence of the delocalized nonradiative surface waves can be transferred into radiative fields by introducing a shallow grating on the surface. We show that the coherency of the thermal radiation can be enhanced by an order of magnitude compared with the coherency imposed by the delocalized surface waves. The enhanced coherency is due to coherent coupling between resonant cavities obtained by surface standing waves, where each cavity supports localized field that is attributed to coupled surface waves. We realized coupled resonant cavity structure on amorphous SiO 2 and crystalline SiC, both support surface phonon-polaritons, to demonstrate extraordinary coherent thermal emission with a high quality factor of 600 and a spatial coherence length of 760λ (8.8mm). Keywords: Resonators, surface waves, thermal emission 1. INTRODUCTION Surface waves have been shown to play a key role in spontaneous thermal emission in the near-field, and dramatically affect the local density of states (DOS) in the vicinity of an interface, as well as the coherence properties of the nonradiative field [1-8]. The spatial coherence of the thermally excited electromagnetic fields in the near-field was investigated by Carminati and Greffet [7]. It has been shown that thermally excited delocalized surface waves yield a long-range spatial coherence length, L c , on a scale of the surface wave propagation length, L || , which can be much larger than the emitted wavelength, L c ≈ L || >> λ [7,8]. The near-field coherence of the delocalized nonradiative surface waves was transferred into radiative fields by introducing a shallow grating on the surface [9]. Such a grating introduced a smooth perturbation on the surface while maintaining the dynamics of the delocalized surface waves. It was found that the coherence length of the coupled radiative fields was limited by the spatial coherence of the delocalized surface waves (l c ≤ L c ) [9,10]. In light of these results, we address the issue whether it is possible to achieve a coherence length of the radiative fields substantially larger than the coherence of the delocalized surface waves, such that (l c >> L c ). The physical phenomena associated with coupled resonant cavities raises the possibility of modifying and controlling well-known surface excitations such as surface plasmons and surface phonon-polaritons (SPPs) [11,12]. In this paper, we experimentally observed and analyzed extraordinary coherent thermal radiation that overcomes the coherence limitation imposed by delocalized surface waves. This resonant enhancement is due to coherent coupling between resonant cavities obtained by surface standing waves, in which each cavity supports standing wave-coupled surface polaritons. Coupled resonant cavities (CRCs) increase the photonic DOS due to thermal excitation of localized surface mode at the resonant frequency supported by the cavities, along with a coupling of the resonant mode into a radiative field. As a result, the spatial coherence length is increased by an order of magnitude compared with the coherence length of the delocalized surface waves. The resonant frequency of the radiative field is strongly dependent on the standing wave’s condition inside the cavities and radiates only in a specific direction. In contrast, thermal emission obtained by coupling of delocalized surface waves has a rainbow-like frequency behavior, i.e., each frequency is emitted in a different direction, one that satisfies momentum conservation [9]. To implement the enhanced coherency, we studied the thermal radiation of CRC structures etched on an amorphous fused silica (SiO 2 ) substrate and SiC. A quasi-monochromatic and highly directional thermal emission in the normal direction was obtained for TM polarization state due to SPPs excitation. The spatial coherence of the radiative field was Photonic Crystal Materials and Devices VII, edited by Ali Adibi, Shawn-Yu Lin, Axel Scherer, Proc. of SPIE Vol. 6901, 69010A, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.761205 Proc. of SPIE Vol. 6901 69010A-1 2008 SPIE Digital Library -- Subscriber Archive Copy